There are a multitude of devices and methods for measuring characteristics of coins and for identifying or recognising them with respect to legal tender which use inductive, capacitive, optical, electromagnetic, acoustic or piezoelectric sensors, aimed at obtaining the physical characteristics of legal tender, such as dimensional, mechanical and especially electrical and magnetic characteristics.
In the case of magnetic sensors, it is normal to use pairs of inductors arranged facing each other, between which the coins are made to pass. The inductors are formed by one or more coils that are introduced in a magnetic core to increase the intensity of the magnetic flow that reaches the coin and the opposing inductor. In order to achieve a sufficiently reliable measurement of the coins, it is normal to use more than one pair of inductors with different configurations, for example using inductors with an in-phase, out-of-phase or emitter-receiver configuration. Alternatively, the individual measurements of each of the inductors and those corresponding to the mutual configuration (emitter-receiver) can be obtained. In any of the aforementioned configurations, an important characteristic is the frequency at which the inductors work, which can vary between a few kilohertz and several megahertz, which determines different depths of penetration of the electromagnetic field in the coin. The depth of penetration of the field generated by the inductors decreases as their frequency increases and, likewise, decreases when the conductivity of the coin or its magnetic permeability increases. This characteristic is important for multi-layer coins, composed by three or more layers of different metals, a structure that represents a secure and useful characteristic for its recognition without interferences with other legal tender.
Document EP0336018B1 discloses a sensor with emitter-receiver configuration wherein an emitting inductor is used that is fed by a periodic signal with high harmonic content. Two inductors are used in the manner of a receiver, each of which is tuned to a frequency, either by means of a resonator or by means of a bandpass filter. In this manner, measurements of the attenuation of the signal emitted in two different frequencies are obtained from the passage of the coin between the emitter and the receiver. The described configuration has the drawback of using three inductors and also only operates in one mode, emitter-receiver, and is therefore not optimal, since it does not measure the impedances inherent to each of the inductors or for generating the in-phase and out-of-phase modes.
Patent EP0886247B1 proposes a sensor with two inductors arranged facing each other between which the coins are made to circulate, wherein the emitter-receiver configuration is used. The emitter uses a periodic signal with harmonic content, such as for example a square signal and the signal provided by the receiver is sampled in defined intervals that are related to the frequencies of the harmonics of the excitation signal. The analysis obtained is of the multi-frequency type by means of a single receiver and has the drawback that it only uses one operating mode and does not have individual inductor impedance measurements, which provide important information to achieve a good discrimination of genuine and counterfeit coins, either through direct information or by calculating the in-phase and out-of-phase modes.
Patent EP1172772A2 discloses a coin sensor wherein two pairs of inductors are used, one of which operates in the in-phase configuration and the other in out-of-phase configuration. Both are fed by a multi-frequency signal, in this case with the addition of three signals of different frequencies that enable analysis of the coins on the surface (high frequency), with partial penetration (intermediate frequency) and total penetration (low frequency). In this sensor, the emitter-receiver mode is not used and, furthermore, the use of two pairs of sensors makes its complexity and required space excessive.
Another example of a multi-frequency sensor can be found in publication EP2203902B1, wherein the inductor forms part of a Maxwell bridge which is fed by a pseudo-random broadband signal. In the absence of coins the bridge is sensitively balanced and, in the presence of coins, a signal appears which, once amplified, is sampled and the representative parameters of the transfer function are obtained, which will be those used to validate the coin. The proposed sensor only has one inductor, due to which it is not possible to obtain parameters related to mutual or emitter-receiver impedance.
Lastly, document EP1445739B1 discloses a method and the corresponding device wherein the coin sensor is composed of two inductors arranged facing each other between which the coin passes and the characteristics of the coin are measured, obtaining measurements of both mutual and individual impedances, which makes it possible to calculate parameters related to the three possible modes: in-phase, out-of-phase and emitter-receiver. Additionally, it has coin lift-off compensation (i.e. the change in the distance between the sensors and the coin along its travel path between the sensors, since the coin does not normally circulate to a constant position with respect thereto); however, all this is done at a single frequency, which is not ideal for a multi-layer coin sensor.